In the manufacture of tennis strings, or strings for other athletic rackets, such as badminton, squash and the like, as well as strings for musical instruments, it is important that the string have the proper combination of elongation (modulus of elasticity) and resiliency (internal damping). A string having too high a modulus of elasticity will not elongate acceptably and hence will not give the desired results; while, by the same token, a string which has high internal damping will not exhibit sufficient resiliency to give the desired results.
It is well known in the manufacture of such strings to provide a core or filler which may comprise a plurality of thermoplastic strands twisted or otherwise bundled together and then integrating said core or filler by coating same with a thermoplastic formulation. In some cases, the integrated core or filler is then provided with some kind of a wrap or sheath thereover, after which the wrapped core is again integrated by coating same with said thermoplastic formulation. Applicant's prior U.S. Pat. No. 2,649,833; 2,712,263; and 3,745,756 are illustrative of tennis strings so manufactured.
It has now been found that the integration of the above described type of string by the aforesaid thermoplastic formulation locks the string, and particularly the core or filler thereof, so tightly together that elongation of the string appears to be decreased and internal damping (energy lost to friction on impact) increased, both of which are undesirable. It has been found, however, that by coating the core or filler with a lubricating agent before applying the thermoplastic formulation to the string, the core or filler is capable of some degree of movement when the string is set into vibration upon contact thus permitting more effective elongation of the string and, at the same time, enhanced resiliency, since there is less dissipation or loss of energy due to friction. The problem, however, has been to find a lubricating agent, or class of lubricating agents, which will permit these desired objectives to be achieved, but which at the same time may be effectively bonded to the thermoplastic formulation which is subsequently applied to the string. In this connection, it has been found that cationic lubricating agents permit the desired bonding to take place with the thermoplastic formulation, while at the same time permitting the desired movement of the core or filler, and its constituent fibers, thereby permitting an optimum degree of resiliency of the filler elements to be achieved.
Other objects, features and advantages of the invention will become apparent as the description thereof proceeds when considered in connection with the accompanying illustrative drawings.